Aromatic Compounds (Chapter 25) Flashcards
What is the molecular formula of benzene?
C6H6
Describe benzene
- Colourless, sweet-smelling and very flammable liquid
- Carcinogen
- Aromatic hydrocarbon/arene
Describe the Kekulé model of the structure of benzene
Based on a six-membered ring of carbon atoms joined by alternative single and double bonds
What was the problem with Kekulé’s model of benzene?
The structure was not able to explain all the chemical and physical properties of benzene
What were the three pieces of evidence used to disprove Kekulé’s model of benzene?
1) the lack of reactivity of benzene
2) the lengths of the carbon-carbon bonds in benzene
3) hydrogenation enthalpies - benzene was more stable than expected
How did the lack of reactivity of benzene disprove Kekulé’s model?
1) if benzene contained the C=C bonds in Kekulé’s model, it should decolourise bromine in an electrophilic addition reaction
2) however, benzene does not undergo electrophilic addition reactions or decolourise bromine under normal conditions
3) ∴ this suggested that benzene does not have any C=C bonds in its structure
How did the lengths of the carbon-carbon bonds in benzene disprove Kekulé’s model?
1) using x-ray diffraction, it was found that all the bonds in benzene were the same length (0.139nm)
2) this bond length was between the length of a single bond (0.153nm) and a double bond (0.134nm)
How did hydrogenation enthalpies disprove Kekulé’s model of benzene?
1) if benzene had the Kekulé structure, it would be expected to have a deltaH of hydrogenation that is three times that of cyclohexane (-120KJ/mol) due to the three double bonds (∴ -360KJ/mol)
2) however, the actual deltaH of hydrogenation of benzene is only -208KJ/mol ∴ 152KJ/mol less energy is produced than expected
3) ∴ the actual structure is more stable than Kekulé’s model
Describe the structure and bonding of benzene (delocalised model)
1) benzene is a planar, cyclic, hexagonal hydrocarbon containing 6 carbon atom and 6 hydrogen atoms
2) each carbon atom uses 3 out of its 4 available electrons in bonding to two other carbon atoms and to one hydrogen atom
3) each carbon atom has one electron in a p orbital at right angles to the plane of the bonded carbon and hydrogen atoms
4) adjacent p orbital electrons overlap sideways, in both directions, above and below the plane of carbon atoms (plane of benzene ring) to form a ring of electron density
5) this overlapping of p orbitals creates a system of pi bonds which spread over all of the 6 carbon atoms in the ring structure
6) the 6 electrons occupying this systems of pi bonds are delocalised
Describe the naming of monosubstituted benzene rings
- Benzene is often the parent chain
- Alkyl groups, halogens and nitro (NO2) groups are prefixes to benzene e.g. ethyl/nitro/chlorobenzene
- When a benzene ring is attached to an alkyl chain with a functional group or one that has more than 7 carbon atoms, benzene is the substituent ∴ the prefix phenyl is used e.g. phenylethanone, phenyloctane
What are the three exceptions to the naming rules of substituted benzene?
Benzoic acid, phenylamine, benzaldehyde
Describe the naming of benzene compounds with more than one substituent group
- The ring is numbered, starting with one of the substituent groups
- The substituent groups are listed in alphabetical order, using the smallest number possible
- e.g. 2-bromomethylbenzene
Describe briefly how benzene reacts
- Benzene and its derivatives undergo substitution reactions in which a hydrogen atom on the benzene ing is replaced by another atom or group of atoms
- Benzene typically reacts with electrophiles and most of the reactions of benzene proceed by electrophilic substitution
- By-product: H+
Describe the nitration of benzene
- Benzene reacts slowly with HNO3 to form nitrobenzene
- The reaction is catalysed by H2SO4 and heated to 50 degrees to obtain a good rate of reaction
- A water bath is used to maintain a steady temperature bc if the temperature rises to more than 50 degrees, further substitution may occur, leading to the production of dinitrobenzene or TNT
- By-product: H2O
What is nitrobenzene used for?
It is used as a starting material in dyes and pharmaceuticals
What is the mechanism by which benzene reacts?
Electrophilic substitution
Describe the mechanism of electrophilic substitution in the context of the nitration of benzene
1) the electrophile (NO2+ - nitronium ion) accepts a pair of electrons from the benzene ring to form a dative covalent bond
2) the organic intermediate formed is unstable and breaks down to form the organic product (nitrobenzene) and the H+ ion - a stable benzene ring is reformed
How is the nitronium ion produced and then how is the catalyst reformed?
1) NO2+ is produce by the reaction of concentration nitric acid and concentration sulfuric acid:
HNO3 + H2SO4 => NO2+ +HSO4- + H2O
2) the H+ ion formed reacts with the HSO4- ions to regenerate the catalyst (H2SO4):
H+ + HSO4- => H2SO4
When, and only when, to halogens react with benzene?
Only when a halogen carrier (catalyst) is present - otherwise they do not react
What are examples of halogen carriers and how are they generated?
AlCl3, FeCl3, AlBr3 - they can be generated in situ from the metal and the halogen
Describe the bromination of benzene
- At room temperature and pressure, in the presence of a halogen carrier, benzene reacts with bromine in an electrophilic substitution reaction to form bromobenzene
- By-product: HBr
- Benzene is too stable to react with a non-polar bromine molecule
